MACHOs, or massive compact halo objects, are what we would call a black hole. The term covers more than just black holes, but it’s good to have a point of reference.

WIMPs, or weakly interacting massive particles, are particles that are about 100 times heavier than a proton. The prevailing theory for them is that they help attract normal matter into clumps that can become galaxies and produce stars.

Originally, scientists theorized that all dark matter was just normal matter that was too dim to see. Without dark matter, there would be nothing holding together the stars that make up galaxies. If scientists understood dark matter, perhaps we would all understand more about the cosmos.

There’s also a pretty interesting story about how the universe could be expanding faster than we thought. The problem with reporting on astrophysics, however, is that every time you turn around there is a new theory to take into account.

And this year is no different.

SIMPs v. WIMPs

Remember MACHOs? Well, earlier theories had them holding galaxies together, while newer theories don’t. A recent study of the Andromeda galaxy by the Subaru Telescope basically put an end to that.

Despite the fact that black holes are notoriously hard to detect, astronomers have become very good at observing them. Apparently, there is a set amount of criteria for primordial black holes, and Andromeda has exactly one. That’s not enough to validate the MACHO theory.

If MACHOs don’t hold the universe together, then, what does? Some scientists theorize that WIMPs do, but there is another, newer candidate. They’re called SIMPs, or strongly interacting massive particles.

SIMPs are relatively new to this game. Whereas WIMPs have decades of research, the theory of SIMPs’ existence is only three years old. It was originally proposed by Prof. Hitoshi Murayama and Yonit Hochberg, and recent evidence might prove them right.

SIMPs theoretically came into being in the early history of the universe, along with WIMPs and MACHOs. Unlike WIMPs, SIMPs interact strongly with themselves through gravity. As for normal matter, well, they interact with that pretty weakly.

Murayama proposed that a SIMP is smaller than a WIMP, which would imply that they outnumber their counterpart by a wide margin. Because of those large numbers, they leave a fingerprint on normal matter.

Murayama’s research is compelling, mostly because he thinks that he has found that fingerprint within the Abell 3827 cluster. There, the dark matter lags behind visible matter, which is not very typical. Murayama’s theory is that these interactions between the dark matter in each galaxy are the key. Those interactions slow down the merger of dark matter (which would give us MACHOs), but not normal matter (which gives us stars).

Thus, with SIMPs, the dark matter is kept separated and the normal matter clumps together. This theory overcomes a major failing of the WIMP theory. Mainly, it helps to explain how dark matter is distributed in small galaxies.

The Search for WIMPs Continues

Let’s get something straight. The existence of SIMPs does not rule out the existence and theories behind WIMPs. Scientists are still very, very interested in finding WIMPs. It’s just that WIMPs fall into the long list of theoretical particles that we haven’t found yet.

The more information we get, the less likely it is that WIMPs alone will provide us with a proper answer. Given that, it isn’t so surprising that the SIMPs theory has popped up and we can assume that more theoretical particles will become apparent to astronomers over the coming years.

So far, however, there are no signs of WIMPs actually appearing. And with all of the astronomic research behind them spawning new theories, we may never actually find one.

The study of dark matter is an incredibly deep rabbit-hole, and I’m not sure that we’ll ever see the bottom of it. In my opinion, that’s a good thing, because it means that years of research and discovery are in our future.

What do you think? Will we ever truly understand dark matter?

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